The inverse relationship between arterial blood pressure and blood volume, demonstrated for the newborn puppy, changes during postnatal development to become a direct relationship in the adult dog. Clinical observations in human neonates and adults suggest a similar pattern of changing responses. The purpose of this study is to investigate interrelationships among certain vasoactive substances, known to alter vascular resistance and blood pressure, with changes in blood volume and to identify differences in these interrelationships that occur during development. Acute and chronic experiments in mongrel puppies between birth and 8 weeks and in adult dogs will be performed to evaluate th capacity of vascular tissues during development to synthesize primary prostaglandins (PGD2, PGE2, 6-keto PGF1a, PGF2a and thromboxane) in vitro (radioimmunoassay) and of the lung to synthesize and deactivate these same prostaglandins and activate angiotensin I in vivo (radioimmunoassay). The relative vasoactive roles of prostaglandins and angiotensin in the regulation of blood pressure will be studied by selective inhibition of prostaglandin cyclo-oygenase with Indomethacin, by angiotensin antagonism with Saralasin and by intravenous administration of exogenous PGE2, prostacyclin and angiotensin II. Moreover, the effect of changing blood volume (Cr51 red cells) on prostaglandin synthesis and renin release to alter cardiac output (dye dilution technique) and vascular resistance will be evaluated. Finally, alterations in renal blood flow (radiolabeled microsphere technique), glomerular filtration rate (inulin clearance) and sodium excretion that follow changes in blood pressure will be examined. The results of these studies will be analyzed to identify physiologic responses to changes in blood volume that may be unique to the neonate and mandate, therefore, alternative approaches to the management of hypotension in the human infant to those employed in the adult.